1. Field of the Invention
The present invention relates to an amplification-type solid-state image sensor mainly comprised of phototransistors.
2. Related Background Art
A solid-state image sensor comprised of MIS (Metal Insulator Semiconductor)-type static induction transistors has been disclosed, for example, in Japanese Laid-Open Patent Application No. 60-206063. FIG. 21 of the accompanying drawings shows the general construction of the MIS-type static induction transistors forming the solid-state image sensor.
FIG. 21 shows that an N.sup.- -type semiconductor layer 212 of the reverse conductivity type is formed on a P.sup.- -type semiconductor substrate 211, that an N.sup.+ source region 213 and an N.sup.+ drain region 215 are formed in a spaced relation in the surface layer of the N.sup.- -type semiconductor layer 212, that a source electrode 214 is formed in the source region 213 and that a gate electrode 216 is formed on an insulator layer 217 in the region separating the source region and the drain region. It is to be noted that although not shown in the Figure, the drain region 215 is provided with a drain electrode which is not shown.
With this solid-state image sensor, in order that a charge generated by light is to be stored, a voltage is applied to the gate electrode 216 so that it is reverse biased with respect to the source electrode 214 and the drain electrode and thus no current flows between the source region 213 and the drain region 215. Under such reverse-biased condition, the depletion of charge carriers occurs in the semiconductor layer 212 below the gate electrode 216 forming a channel region so that the positive holes generated within the depletion layer are stored in the surface portion of the semiconductor layer 212 below the gate electrode 216.
When a signal is to be read out, a voltage which is intermediary between those of the charge storage period and the change resetting period is applied to the gate electrode 216 so that a current amplified in accordance with the amount of the holes generated by the light reception flows between the source region 213 and the drain region 215. Also, at this time the holes stored below the gate electrode 216 are maintained therein.
In order that the charge generated by the light is to be reset, a voltage is applied to the gate electrode 216 so that it is forward biased with respect to the source electrode 214 or the drain electrode. When this occurs, the holes stored in the semiconductor layer 212 below the gate electrode 216 are discharged in a short period of time toward the P.sup.- substrate 211 side.
As described hereinabove, owing to such unit structure of the MIS-type static induction transistors as shown in FIG. 21, not only the required operations for the solid-state image sensor, i.e., the photoelectric conversion, the storage of charges, the reading of signals and the resetting of stored charges can be performed but also the resetting operation can be effected at high speeds without the occurrence of resetting noise and moreover the reading of amplified signals can be effected in a nondestructive manner.
However the conventional solid-state image sensor of the above-described construction is disadvantageous in that when storing a charge generated by light, the depletion of charge carriers occurs in the semiconductor layer below the gate electrode in each MIS structure with the result that the dark current generated at the boundary surface between the insulator layer and the semiconductor layer becomes so large that it cannot be ignored and therefore the S/N ratio is deteriorated in cases where the intensity of received light is weak.